Despite many attempts to decrypt it, the final section of the Kryptos sculpture remains unsolved. Jim Sanborn, the sculptor, told the New York Times in 2010 that the 64th to 69th characters, which read NYPVTT, will read BERLIN when decoded.

This week, Mr. Sanborn gave the Times a second clue: the 70th through 74th positions, which read MZFPK, will read CLOCK when decoded.

Twenty years of the best cryptographers in the world having a crack at a 97-letter puzzle, and it remains unsolved. Truly the contemporary Enigma, as it were.